Fault current recognition circuitry

ABSTRACT

A circuitry for detecting a fault current or leakage current on the supply line of an electronic circuit includes a current source which is rated for a maximum allowed fault current or leakage current. The current source is fed by an auxiliary voltage source. The potential on the supply line which prevails when the supply voltage is disconnected due to the leakage current is determined and assessed by a potential monitor.

BACKGROUND OF THE INVENTION

The present invention relates to a circuitry which is used for detectinga fault current on the supply line of an electronic circuit and which isequipped with an auxiliary voltage source and a potential monitor fordetermining and analyzing the potential prevailing on the supply lineduring a testing period.

German patent application No. 42 42 177 discloses a circuitry of thistype which is used to monitor a large number of valve coils and theirassociated final stages. For fault current detection, an auxiliaryvoltage source is connected to the common supply line for the valvecoils by way of a high-ohmic resistor, and the potential on the supplyline is measured and analyzed. Before this action, the connection of thesupply line to the supply source, i.e., the vehicle battery, wasinterrupted by way of a semiconductor relay. The associated potentialmonitor is connected by way of a high-ohmic voltage divider. When ashunt circuit or a leakage current from the supply line to the supplysource or to ground occurs, the potential on the supply line will vary.

An object of the present invention is to provide a circuitry of thistype which has a high degree of accuracy, sensitiveness and reliabilityin the detection of fault currents, which also includes leakagecurrents, and, in addition, can be realized with comparatively littleeffort. A simple and, hence, low-cost measuring apparatus for monitoringthe potential on the supply line is deemed sufficient.

SUMMARY OF THE INVENTION

It has been found that this object can be achieved by the design of acircuitry with the special feature that the installation of a currentsource, which is rated for a maximally allowed current and preferablyincludes two anti-parallel single current sources, prevents response ofthe monitoring means as long as the fault current is within the range ofpredefined limit values, however, that even when the limit values areonly slightly exceeded, a very apparent and, therefore, easilyassessable potential variation on the supply line is caused during thetesting period.

In a preferred aspect of the present invention, two single currentsources responding to fault currents in opposite directions are used,whereby it is ensured that fault currents, or shunt circuits, from thesupply line to the positive pole of the supply source and to ground aredetected.

The circuitry of the present invention is especially appropriate, forexample, for monitoring the valve coils and the associated electroniccircuits of a controlled brake system, wherein the coils are connectedto the supply voltage by way of a common supply line and a common relay.In a brake system of this type, the detection of a fault current must beensured with a high degree of reliability because the control must bedeactivated in a case of malfunction in order to keep the brake systemin function. Therefore, the application is critical with respect tosafety.

Further features, advantages and possible applications of the presentinvention can be seen in the following description of an embodiment,making reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings,

FIG. 1 is a schematically simplified view of the generally electricalcomponents of a circuitry of the present invention.

FIG. 2 is a wiring diagram of a monitoring circuit on the basis of thecircuitry of FIG. 1.

FIG. 3 is a diagram in which the signal variation is plotted as afunction of the fault current in a circuitry of FIG. 1 or FIG. 2.

FIG. 4 is the basic circuit of a potential monitor for the circuitriesof FIG. 1 or FIG. 2.

FIG. 5 is a diagrammatic view of a part of an integrated circuit forrealizing current sources for the circuitry of FIG. 1 or FIG. 2.

FIG. 6 is a variation of the circuit in FIG. 5 in the same illustrationas in FIG. 5.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 is used to illustrate the basic operation of the circuitry of thepresent invention. L1, L2, Ln designate valve coils comprised in thehydraulic valves of an automotive vehicle control system, for example,an anti-lock brake system (ABS), used for braking pressure modulation.The coils are activated and deactivated by switches or final stages T1,T2, Tn. For energy supply, a joint supply line REF is connected by arelay Rell with a working contact K₁ to a supply source, i.e., to avehicle battery having the voltage U₃. Contact K₁ is made and anti-lockcontrol is operating only in the absence of faults.

A monitoring circuit 1 is provided to detect a fault current. Only acurrent source IQ including two single current sources -IQ, +IQ is shownin FIG. 1. The monitoring circuit 1 is connected to an auxiliary voltagesource U_(H). Because the potential of the auxiliary voltage sourceU_(H) is lower than the potential of the battery voltage U_(B) andhigher than the joint ground potential GND, fault currents in oppositedirections are possible in a shunt circuit from the supply line REF tothe ground potential GND, on the one hand, and in a shunt circuit fromthe supply line REF to the battery voltage U_(B). Therefore, the currentsource IQ in the embodiment shown has two anti-parallel connected singlecurrent sources -IQ and +IQ. When Rell or K₁ is open, and with openswitches T1, T2, Tn, current flow is caused by way of the signal currentsource -IQ in the event of a shunt circuit from the supply line REF toground GND and by way of the single current source +IQ in the event of ashunt circuit to the supply voltage +U_(B). The potential on the supplyline REF is (almost) maintained on the potential U_(H) (as will beexplained in the following with reference to FIG. 3), but only as longas the amount of the fault current is inferior to a predetermined limitvalue, or only as long as the fault current ranges between predeterminedlimit values±I_(max). "I_(max) " and "-I_(max) " are the "nominalcurrents" of the current sources "+IQ" or "-IQ".

When the amount of the fault current rises in excess of these limitvalues±I_(max), this will cause a steep variation of the potential onthe supply line REF. This potential variation is signaled to the inputof a potential monitor PM1 by way of a voltage divider R₁, R₂. Thepotential variation is analyzed by way of the potential monitor PM1 anda microcomputer MC which performs still further testing and monitoringoperations (not described). As soon as a fault is detected, the relayRell is caused to open by way of a signal line 2, or closing of contactK₁ of the relay is prevented. This way, the valves which include coilsL1, L2, Ln are prevented from responding. This deactivates the control.

FIG. 1 and FIG. 2 show the same circuitry. FIG. 1 illustrates only thosecomponents which are important for the present invention, whereas FIG. 2shows further details.

Final stages for activation and deactivation of the exciter current forthe valve coils L1 to L4 are represented in FIG. 2 by switches T1 to T4which are actuated by terminals G1 to G4. It is determined bycomparators TH1 to TH4 whether the potential at the output of theswitches or transistors T1 to T4 is above or below a predetermined limitvalue which is defined by way of the auxiliary voltage source U_(H) anda voltage divider R₃, R₄. When switch T1 to T4 is opened, the potentialis determined in the illustrated embodiments by current sources Q1 to Q4at the corresponding input of comparators TH1 to TH4.

The output levels of the comparators TH1 to TH4 are registered by ashift register SR1 and conducted serially to the microcomputer MC.Accordingly, test pulses are sent from the microcomputer MC via aconverter 3 to the control terminals G1 to G4 of the switches ortransistors T1 to T4. As has already been explained by way of FIG. 1,the valve coils L1 to L4 are connected to the voltage U_(B) of thevehicle battery by way of relay Rell.

The potential on the supply line REF is determined and monitored bymeans of two potential monitors PM1 and PM2 which are connected to thesupply line REF by way of voltage dividers. The function of thepotential monitor PM1 has already been described with reference to theembodiment of FIG. 1. The second potential monitor PM2 is a windowcomparator having a commutable threshold. A switch TH5 switches thethreshold over. The output signal of the comparator PM2 is converted ina control unit ST1 to a signal for actuating the semiconductor relayRell. An output of the microcomputer MC extends to the control unit ST1by way of a line 2'. When a defect is detected, more particularly, afault current, Rell is deactivated by way of the control unit ST1, andcurrent supply of the valve coils L1 to L4 or L1 to Ln is therebyinterrupted.

Another comparator TH6 is used to determine the switching condition ofthe relay Rell. The operation of the circuitry of FIG. 2 is as follows:

Relay Rell is the main relay. When the ignition is actuated, initially,contact K₁ of Rell is made. The potential variation on the supply lineREF caused thereby is monitored by the potential monitor PM2. As long asthere is no shunt, and contact K₁ is open, the potential on the supplyline REF is determined by the auxiliary voltage source U_(H). After K₁is closed, the battery voltage U_(B) will prevail on the supply line REFin relation to ground. The amount of U_(B) is substantially higher thanU_(H).

After contact K₁ is closed, the voltage on the input of monitor PM2 willrise in excess of the threshold voltage on the second input of theillustrated comparator of the monitor PM2. The threshold voltage ispredefined by the voltage U_(H) and the resistors R₈, R₉. The output ofthe comparator switches to "high". Subsequently, the threshold of thecomparator is set to a higher value by way of switch TH5 so that theoutput of the comparator becomes "low" again. The function of themonitor PM2, with proper operation of relay Rell, is simultaneouslytested by these low-high-low transitions. After the switch-over to thehigher threshold, the comparator of monitor PM2, by way of the controlunit ST1, is used to activate and, if necessary, deactivate the relayRell when excessive voltages occur on the supply source U_(B). Theactivation of the valve coils L1 to L4 is tested by way of short testpulses. The respective switch T1 to T4 is closed by these test pulses,and the potential variation caused thereby on the input of thecorresponding comparator TH1 to TH4 is assessed in the fashion describedhereinabove. The contact K₁ of relay Rell is open during these testpulses.

Monitoring the supply line REF with respect to shunts to the supplyvoltage source +U_(B) or to ground is performed in the fashion describedhereinabove by means of the current source IQ or the single currentsources +IQ and -IQ.

The embodiment of FIG. 3 shows the potential variation on the supplyline REF as a function of the shunts or the corresponding fault currentsI_(FS). The voltage of the auxiliary voltage source U_(H) amounts to 5Vin the present case. The current source IQ is rated ±10 mA in thisembodiment. As long as the fault current is within the limits ±10 mA,the potential on the supply line REF is maintained on the potentialU_(H) =5V by the voltage source IQ. However, as soon as the currentexceeds the value of ±10 mA (even if this exceeding is onlyinsignificant), the potential will change almost abruptly in thedirection of ground potential or in the direction of the battery voltageU_(B). A very simple potential monitor PM1 is sufficient to determineand assess this potential variation. The respective tolerances, forexample, of the voltage divider resistors R1, R2, the actual amount ofthe battery voltage U_(B), etc., are not important for the accuracy andreliability of the fault current detection.

A simple example of a potential monitor of this type is shown in FIG. 4.The potential on the supply line REF is compared with predeterminedthreshold values U_(max) and U_(min) by way of two comparators 4, 5. Afault current identification signal is issued by an OR gate 6 when thethreshold values are reached.

FIG. 5 shows an example where the current source IQ is realized by meansof an integrated circuit which includes current mirror circuitries.

A defined reference current "IREF" is adjusted in a known manner by wayof an external resistor "R_(IREF) " and an internal reference voltage"IREF". This circuit is connected to the external auxiliary voltagesource U_(B). A reference quantity BIAS1 is produced in a known fashionby way of a current mirror circuit with the transistors Sp1, Sp2, Sp3.BIAS1 is applied to the basis of the transistors Sp4, Sp5 and determinesthe operating point of the transistors. A single current source -IQ isprovided by a current-mirroring operation by means of the transistorsSp6, Sp7 which are connected to the collector of the transistor Sp4 andrepresent a current mirror circuit. The single current source +IQ isprovided by way of the serially connected transistors Sp5, Sp8. Acurrent source IQ is thereby provided which is composed of twoanti-parallel connected current sources and can furnish a current of apredefined magnitude to a higher potential (+U_(B)) or to a lowerpotential (GND). The nominal value of these current sources, i.e. themaximum value (+I_(max), -I_(max)) for which the current sources arerated, may be equal in both directions (as in the present embodiment),or different maximum values may be predefined.

Integrated circuits of this type are known in the art. The relativelygreat difference in voltage upon exceeding the predetermined currentlimit values +I_(max) or -I_(max) reduces the demands placed on thepotential monitor and makes the monitoring system less sensitive tostray fields (electromagnetic compatibility). The total structure neededfor the circuitry of the present invention is thus comparatively small.

The embodiment of FIG. 6 shows a circuit design variation which differsfrom the circuit of FIG. 5 only by the slightly different connection ofthe transistors Sp9, Sp10, Sp11 which form the right-hand current source+IQ. This circuit variation shows that the voltage sources -IQ and +IQdiffers only by exchange of the connections A1, A2 leading to theauxiliary voltage source U_(H) and the REF connection. The currentsources -IQ and +IQ are identical in terms of their circuits, however,the directions of current flow are contrary, due to exchangedconnections A1, A2.

We claim:
 1. A circuitry for detecting a fault current on a supply lineof an electronic circuit, including an auxiliary voltage source and apotential monitor for determining and analyzing the potential prevailingon the supply line during a testing period, wherein the supply line isconnected to the auxiliary voltage source by way of a current sourcewhich is rated for a maximum allowed fault current, wherein thepotential of the auxiliary voltage source ranges between the potentialof a supply voltage and a ground potential, and wherein the currentsource includes two anti-parallel connected single current sources whichare each rated for the maximum allowed fault current of one direction.2. A circuitry as claimed in claim 1, further including circuitry meansfor monitoring valve coils and associated electronic circuits of acontrolled brake system, wherein the coils are connected to the supplyvoltage by way of a common supply line and a common relay.